Various rubber compositions for components for various products, such as for example tires, contain particulate reinforcement comprised of a combination of precipitated silica and rubber reinforcing carbon black together with a coupling agent for the silica. Such rubber compositions also conventionally contain a combination of zinc oxide and stearic acid additives.
Various coupling agents have been proposed for coupling the precipitated silica to the diene-based elastomer for such rubber compositions.
Historically, coupling agents have heretofore been proposed which contain one moiety reactive with a precipitated silica (e.g. silanol groups on the silica) and another different moiety interactive with an unsaturated conjugated diene-based elastomer. Such coupling agents may be for example, a bis(3-triethoxysilylpropyl) polysulfide which contains an average of from about 2 to about 4 connecting sulfur atoms in their polysulfidic bridge and, for example, alkoxyorganomercaptosilanes in which at least a portion of its alkoxy moiety is comprised of ethoxy groups.
The alkoxyorganomercaptosilane silica coupling agent may have its mercapto moiety chemically capped, or blocked, to retard promoting an increased viscosity buildup in the preliminary, non-productive, mixing of the rubber composition. The rubber composition conventionally contains an unblocking agent to unblock the chemically blocked mercapto moiety subsequent to the aforesaid non-productive mixing stage to enable the coupling agent to interact with a diene-based elastomer in the rubber composition. Such unblocking agent may be, for example, an amine-containing sulfur cure accelerator added in a non-productive or productive mixing stage.
Such coupling agents typically contain an ethoxy moiety to react with the hydroxyl groups on the precipitated silica and release ethanol as a by-product.
Such coupling agents and their use in precipitated silica reinforcement-containing rubber compositions are well known to those having skill in such art.
In one aspect, such organomercaptosilane based coupling agent may be an oligomeric organomercaptosilane (organomercaptosilane oligomer) comprised of plurality of mercapto and silane moieties which contains only a minimal alkoxy group content, if any, particularly only a minimal, if any, alkoxy groups in a form of an ethoxy group, therefore being substantially free of ethoxy groups (and thereby does not release an ethanol by-product to any significant extent upon reaction with hydroxyl groups, for example, silanol groups, of a precipitated silica).
In another aspect, such organomercaptosilane oligomer may also, if desired, have only a portion, of any, of its plurality of mercapto moieties chemically blocked as earlier described in a sense that the unblocked mercapto moieties may interact with diene-based elastomers in the associated rubber composition in a preliminary, non-productive, rubber mixing stage in an internal rubber mixer to thereby increase the mixing viscosity (Mooney viscosity) and cause the processing (e.g. mixing) of the rubber composition to be significantly more energy demanding and difficult.
A challenge is seen herein to utilize such organomercaptosilane oligomer with its plurality of mercapto groups as a coupling agent in a manner which does not excessively increase the mixing viscosity of the rubber composition. A further challenge is to utilize the oligomeric organomercaptosilane in such manner which can also yield a resultant rubber composition with suitable physical properties.
As hereinbefore indicated, diene-based elastomer compositions typically contain a combination of zinc oxide and long chain carboxylic (fatty) acid such as, for example, stearic acid. The combination of zinc oxide and fatty acid (e.g. stearic acid) eventually forms a zinc fatty acid salt (e.g. zinc stearate) in situ within the rubber composition, although it is envisioned herein that a significant portion of the fatty acid (e.g. stearic acid) remains in its fatty acid (e.g. stearic acid) form during a significant portion of the aforesaid non-productive mixing step.
For this invention it has been found unexpectedly, that by a combination of adding the zinc oxide in a non-productive rubber mixing stage, or step, instead of a subsequent productive rubber mixing stage, together with an addition of a greater amount of the fatty acid (e.g. stearic acid) in a sense of equal or greater than equal weight amount of the fatty acid (e.g. stearic acid) relative to the zinc oxide in the non-productive mixing stage of the silica-containing rubber mixing process which contains an inclusion of an oligomeric organomercaptosilane coupling agent with a significant portion, particularly at least 30 percent, alternatively at least 50 percent, of its mercapto groups being unblocked, a significant viscosity buildup of the rubber composition is inhibited, or retarded, and a suitable viscosity (e.g. Mooney viscosity) of the unvulcanized rubber composition can be obtained.
A further challenge is presented in a sense that the presence of the sulfur contained in the unblocked mercapto groups for the oligomeric organomercaptosilane coupling agent adds to the presence of the sulfur curative in the rubber composition in a manner that a reduction in the addition of sulfur curative (free sulfur) is, and would be expected to be, necessary to provide suitable crosslink density for the sulfur cured rubber composition as may be evidenced, for example, by its ultimate elongation at break, as well as other desired physical properties.
Although the mechanism is not fully understood, it is envisioned herein that the aforesaid addition of a greater amount of fatty acid (e.g. stearic acid), (equal to or greater than the weight of the zinc oxide) added in the non-productive mixing stage (together with the zinc oxide in a non-productive stage) interacts with the organomercaptosilane oligomer, particularly its unblocked mercapto moieties, to retard the subsequent sulfur curing of the resultant rubber composition and/or coupling of the silica to the diene-based elastomer(s) and thereby negatively affecting various resultant physical properties for the sulfur cured rubber composition, including the sulfur crosslink density of the cured rubber composition as indicated by its ultimate elongation (elongation at break), a well known physical property to those having experience in such art.
Accordingly, it has further been discovered that while such aforesaid controlled zinc oxide and fatty acid, particularly stearic acid addition in the non-productive mixing step(s) can enable a preparation of a precipitated silica reinforced rubber composition having a Mooney viscosity which is suitably processable in its uncured condition, it has further been discovered that such phenomenon, when combined with an increase, rather than a decrease, in addition of sulfur curative (e.g. by not reducing the addition of sulfur curative) can enable a production of a precipitated silica reinforced rubber composition which is both suitably processable (e.g. having a suitable Mooney viscosity) in its uncured condition but also has an improved crosslink density and various suitable physical properties in its subsequently sulfur cured condition.
Accordingly, it is considered herein that this aspect of the invention does not rely upon simply adding sulfur to obtain an increased crosslink density for the sulfur cured rubber composition in a vacuum, but, instead, relies upon first recognizing the aforesaid problem caused by the additional fatty acid, particularly stearic acid, additive interacting with the plurality of mercapto groups of the oligomeric organomercaptosilane in the non-productive mixing stage to cause a reduction in crosslink density combined with solving the problem by the addition of a greater than expected amount of sulfur curative.
From an historical perspective, a long chain carboxylic (fatty) acid for use in preparation of rubber compositions is typically stearic acid which, in practice, is typically comprised of at least about 90 weight percent stearic acid and minor amounts (less than 10 weight percent) of other long chain carboxylic fatty acids typically including palmitic acid and oleic acid. For this invention, such fatty acid is referred to as being “stearic acid” even though it is considered herein as typically being somewhat impure and being comprised of at least 90 weight percent stearic acid and less than 10 percent of other long chain fatty acids such as for example, primarily palmitic and oleic acids.
Alternatively, a long chain carboxylic acid (fatty acid) may be comprised of, for example, at least one of fatty acid derivatives comprised of fatty acid polyol, fatty acid ester, fatty acid glyceride, fatty acid amide, fatty acid amino acid, and zinc fatty acid soap, and their combinations.
In practice, sulfur vulcanized elastomer products are typically prepared by thermomechanically mixing rubber and various ingredients in a sequentially step-wise manner followed by shaping and curing the compounded rubber to form a vulcanized product.
First, for the aforesaid mixing of the rubber and various ingredients, typically exclusive of sulfur and sulfur vulcanization accelerators, the elastomer(s) and various rubber compounding ingredients are typically blended in one or more thermomechanical mixing stage(s) in suitable mixers, in the absence of sulfur and sulfur vulcanization accelerator(s), which is generally referred to as “non-productive mixing steps”. Such non-productive mixing is usually conducted at elevated temperatures within a range of about 140° C. to 190° C. and often within a range of about 150° C. to 180° C.
Following and subsequent to such non-productive mixing stage, or stages, the rubber composition is mixed in a final mixing stage, sometimes referred to as a “productive mix stage”, where sulfur and sulfur vulcanization accelerators (curatives), and sometimes optionally one or more additional ingredients, are mixed with the rubber composition, typically at a significantly lower temperature in a range within about 100° C. to about 120° C., which is a lower temperature than the temperatures utilized in the non-productive mix stages in order to prevent or retard premature curing of the sulfur curable rubber, which is sometimes referred to as scorching, of the rubber composition.
The rubber mixture, or composition, is typically allowed to cool, sometimes before or after intermediate mill mixing of the rubber composition, between the aforesaid various mixing steps, for example, to a temperature below 50° C.
Such sequential non-productive mixing steps, including the intermediary mill mixing steps and the concluding final productive mixing step are well known to those having skill in the rubber mixing art.
By thermomechanical mixing, it is meant that the rubber compound, or composition of rubber and rubber compounding ingredients, is mixed in a rubber mixture under high shear conditions where the mixture autogeneously heats up, with an accompanying temperature rise, as a result of the mixing primarily due to shear and associated friction within the rubber mixture in the rubber mixer.
One and often two or more of such aforesaid sequential non-productive (NP) mixing stages, or steps, usually in an internal rubber mixer, at elevated temperatures are used, followed by an aforesaid productive (PR) mixing stage at a lower temperature.
This invention is focused on the use of an oligomeric organomercaptosilane based coupling agent for a silica (e.g. precipitated silica)-containing diene-based elastomer rubber composition in combination with the use of an aforesaid specified addition of a combination of zinc oxide and fatty acid comprised of stearic acid.
It is considered herein that a significant aspect of this invention is the use of an oligomeric organomercaptosilane (which contains a plurality of mercapto and silane moieties, with a portion of the mercapto moieties being optionally chemically blocked) in combination with specific amounts and controlled introduction of zinc oxide and stearic acid in a non-productive mixing stage(s) in silica-containing (e.g. precipitated silica) diene-based elastomer rubber compositions. A further aspect is the use of an adjusted sulfur addition in the subsequent productive mixing stage.
It is considered herein that such aspect of this invention involves use of abnormal amounts of the stearic acid in conjunction with more normal amounts of zinc oxide wherein both the zinc oxide and stearic acid are added to the rubber composition in at least one non-productive mixing stage, coincidentally with, or preferably prior to the addition of the oligomeric organomercaptosilane and, also, preferably coincidentally with or prior to the addition of the precipitated silica.
It is considered herein that such aspect of the invention is therefore a significant departure from use of more conventional preparation of silica-containing rubber composition.
The term “phr” as used herein, and according to conventional practice, refers to “parts of a respective material per 100 parts by weight of rubber, or elastomer”.
In the description of this invention, the terms “rubber” and “elastomer” if used herein, may be used interchangeably, unless otherwise prescribed. The terms such as “rubber composition”, “compounded rubber” and “rubber compound”, if used herein, are used interchangeably to refer to rubber which has been blended or mixed with various ingredients and materials and “rubber compounding” or “compounding” may be used to refer to the mixing of such materials. Such terms are well known to those having skill in the rubber mixing or rubber compounding art.